gotten and where it is going now, and to point out some directions for the
future.
The rest of this chapter attempts to describe in informal terms a number of
the models which have been proposed in previous work and to show that
the approach is indeed a fruitful one .It also contains a brief description of the
major sources of the inspiration we have obtained from the work of other
researchers.
Examples of PDP Models
In what follows, we review a number of recent applications of PDP models to
problems in motor control, perception, memory, and language .In many cases,
as we shall see, parallel distributed processing mechanisms are used to provide
natural accounts of the exploitation of multiple, simultaneous, and often mu-
tual constraints .We will also see that these same mechanisms exhibit emergent
properties which lead to novel interpretations of phenomena which have tra-
ditionally been interpreted in other ways.
Motor Control
Having started with an example of how multiple constraints appear to oper-
ate in motor programming, it seems appropriate to mention two models in
this domain .These models have not developed far enough to capture the full
details of obstacle avoidance and multiple constraints on reaching and grasp-
ing, but there have been applications to two problems with some of these
characteristics.
Finger Movements in Skilled Typing One might imagine, at first glance, that
typists carry out keystrokes successively, first programming one stroke and
then, when it is completed, programming the next .However, this is not the
case .For skilled typists, the fingers are continually anticipating upcoming key-
strokes .Consider the wordvacuum .In this word, thev,a,andcare all typed
with the left hand, leaving the right hand nothing to do until it is time to type
the firstu .However, a high speed film of a good typist shows that the right
hand moves up to anticipate the typing of theu, even as the left hand is just
beginning to type thev.Bythetimethecis typed the right index finger is in
position over theuand ready to strike it.
When two successive key strokes are to be typed with the fingers of the same
hand, concurrent preparation to type both can result in similar or conflicting
instructions to the fingers and/or the hand .Consider, in this light, the differ-
ence between the sequenceevand the sequenceer .The first sequence requires
thetypisttomoveupfromhomerowtotypetheeand to move down from the
home row to type thev, while in the second sequence, both theeand therare
above the home row.
The hands take very different positions in these two cases .In the first case,
thehandasawholestaysfairlystationaryoverthehomerow.Themiddlefin-
ger moves up to type thee, and the index finger moves down to type thev .In
thesecondcase,thehandasawholemovesup,bringingthemiddlefingerover
theeand the index finger over ther.Thus,wecanseethatseveralletterscan
simultaneously influence the positioning of the fingers and the hands.
The Appeal of Parallel Distributed Processing 65